1. Field of the Invention
This invention relates to a camera having an electromagnetically operated shutter.
2. Description of the Prior Art
Electromagnetically driven shutters of the type in which shutter blades that also serve as diaphragm blades which are driven to open and close the shutter by means of an electromagnetic drive source of the moving coil type are known. In a shutter of this type, the driving force available is almost unvarying. This results in a uniformly accelerated motion of the moving part such as a sector ring, etc. Therefore, the opening characteristic of the shutter is represented by a curve close to a parabola. Accordingly, the length of time required before an aperture is obtained varies with the degree of overlap of the shutter blades. Hence, in order to have accurate control, it has generally been considered advantageous to obtain a small aperture by establishing a shallow overlap for the shutter blades and by moving the blades more slowly at the smaller diameter portion stage in the process of opening the aperture. In the accompanying drawings, FIGS. 4(a) and (b) show this graphically. The graphs show the relation of time to the rotation angles of the sector rings of electromagnetically operated shutters which have the same opening stroke of the shutter blade members from a pin-hole state to a fully open state while the blade members are arranged to overlap each other to different extents. FIG. 4(a) represents the arrangement where the degree of shutter blade overlap is deep and FIG. 4(b) an arrangement where the degree of shutter blade overlap is shallow. As will be clearly understood from FIGS. 4(a) and (b), the rotation of the sector ring of an electromagnetically operated shutter exhibits a close to uniform acceleration movement, which results in a shutter blade opening characteristic curve close to a parabola. Accordingly, the length of time between a pin-hole state and a fully open state is longer where the degree of blade members overlap is shallow and the energy of motion is small at the pin-hole point as in the case of FIG. 4(b) than where the overlapping degree of blade members is deep as shown in FIG. 4(a). Thus, the arrangement represented by FIG. 4(b) permits more accurate exposure control. In other words, in an electromagnetically operated shutter, the shallower the overlapping degree is, the greater the accuracy thereof will be.
However, in the case of an electromagnetically operated shutter consisting of two blade members, in order to obtain a higher light shielding effect when these blade members are closed, they must be arranged to overlap to a deeper extent. Hence, in the conventional electromagnetically operated shutter of this type, a considerable amount of energy of motion is stored at a moving system (such as the blade members) when these blade members open from a closed condition to the point where they form a pin-hole. Then, the stored energy hinders accurate control over the blade members, particularly in the case of a small aperture. This has resulted in many inconveniences for exposure control. Meanwhile, the electromagnetically operated two-blade shutter is advantageous with respect to cost on account of the smaller number of blade members and parts required, which not only facilitates assembly work but also results in fewer parts that might cause a malfunction. These advantages have recently prompted use of the shutter of this type. Therefore, a strong desire has arisen for an electromagnetically operated shutter that obviates the above-mentioned shortcoming.
Also conventionally known is an electromagnetically operated shutter in which the object brightness is measured in advance through a preliminary aperture with an auxiliary diaphragm. In such conventional shutters, it is common that when the preliminary aperture of the auxiliary diaphragm is once closed by the start of the operation of the blade members of the electromagnetically operated shutter, the count starting switch for exposure control is mechanically turned on to control the exposure by the auxiliary diaphragm, or the count starting switch is turned on by electrically detecting the closure of the preliminary aperture of the auxiliary diaphragm.
It is also conventionally known that the count starting switch is turned on a predetermined time after the current passage to the moving coil of an electromagnetically operated shutter so as to detect the extent of exposure in response to the opening operation of the blade members.
The above conventional electromagnetically operated shutters have a number of defects. Where the count starting switch is mechanically turned on by the operation of the blade member, etc., when the preliminary aperture of the auxiliary aperture of the auxiliary diaphragm is closed, the preliminary aperture is completely shut by the operation of the blade member. Because the count starting switch must be mechanically turned on before the opening of the main aperture is started by the blade members, the timing adjustment is complicated and erroneous.
Further, where the closure of the preliminary aperture of the auxiliary diaphragm is electrically detected, the brightness range in which the camera is operative must be at the light level of 2.sup.10 -2.sup.15 is that quite a large circuit is required for consistent operation of the detection circuit. This causes an increase in the production cost.
Also where the count starting switch is turned on a predetermined time after the current passage to the electromagnetically operated shutter for actuating the blade members, a timer which can operate accurately in a temperature range from -20.degree. C. to 45.degree. C. such as a crystal oscillator is required. This results in a considerable increase in the circuit cost.